DESCRIPTION (Investigator's Abstract): My long term goal is to identify mechanisms of neuronal damage following transient ischemia and provide a basis for evaluating potential treatment for postischemic injury in the CNS. Cerebrovascular disease (stroke) is one of the leading cause of death in the United States. Over 500,000 patients suffer the effects of hypoxic- ischemic' brain damage yearly. Excitotoxic effect of excessive glutamate release during ischemia has been postulated as one of the primary factors in postischemic neuronal damage. However, previous studies fail to provide a consistent picture of neuronal hyperactivity following ischemia with extracellular recording or brain slice preparations. Using in vivo intracellular recording and staining techniques, the objective of the proposed research is to elucidate' changes in neuronal and synaptic activity that occur in pyramidal neurons of rat hippocampus after transient forebrain ischemia, and to determine their roles in selective neuronal injury following- -ischemia. The main hypotheses to be tested are whether neuronal activity of CAl neurons is enhanced after transient ischemia, and whether such enhancement is due to a) enhanced synaptic transmission or b) increased intrinsic excitability of CAl neurons. Secondarily it will be determined whether neurophysiological changes differ in ischemia- sensitive CAl neurons and ischemia-resistant CA3 neurons following, ischemia, thereby identifying those factors that may contribute to the selective vulnerability of CAl neurons. Forebrain ischemia of 5-20 min duration will be induced using the 4-vessel occlusion method. Spontaneous activities and evoked potentials of CAl and CA3 neurons before, during, and at different intervals after transient ischemia will be compared to reveal the postischemic changes of neuronal activity and synaptic transmission. Changes in neuronal excitability will be studied by comparing membrane properties of the neuron before and after transient ischemia. Neurophysiological changes associated with selective neuronal injury will be determined by comparing the postischemic responses of CA I and CA3 neurons. The neurons in which the recordings are made will be identified by intracellular staining with neurobiotin. The proposed experiments will bridge the gap between previous extracellular recording studies of ischemia in vivo and intracellular recording studies of hypoxia/ ischemia in vitro. The results of this study will improve understanding of the mechanisms of brain damage upon resuscitation following cardiac arrest.